Pioneering research by English physicist Michael Faraday led to the discovery of electromagnetic induction in 1830. Using a ring of soft iron, Faraday wound separate coils of wire on opposite ends of the ring. One of the wire coils was to a battery, causing a flow of electric current through the wire coil, magnetizing the iron ring and inducing current into the second wire coil. Current flow into the second coil was controlled with a manually operated switch which opened and closed the circuit, starting and stopping the flow of current.
In 1832, Joseph Henry, an American physicist engaged in experimental work similar to Faraday's by publishing a technical paper describing his own observations in the new found physics of electromagnetic induction. Henry described experiments in which large, bright electric arcs were produced when disconnecting a current carrying electrical cable from a battery. He noted that the wire and the battery terminal (current source) caused the current to continue its flow between the two connections while being pulled apart. The same phenomenon could be observed in a device such as a manual switch. Henry referred to this electric arcing phenomenon as “self-induction”.
Development of the world's first electric generators and motors evolved out of the pioneering research by Faraday and Henry. Commercial production of the first prototype electric generators began in 1850 and motors in 1870. These early machines were powered by direct current (DC). In 1888 the first successful electrical system in America utilizing alternating current (AC) was developed by Nikoli Tesla. The wide use of AC power was furthered by the scientific efforts of early electrical engineers such as George Westinghouse and Charles Steinmetz. Although DC motors and devices continue in use, AC machines are the most prominent devices of choice. From the beginning serious technical problems were encountered in the starting and stopping of AC and DC rotating electric machines.
Research and engineering in the starting and stopping of AC and DC rotating electrical machines has evolved, out of necessity, into a highly specialized area of electrical engineering. Electrical devices for opening and closing motor circuits have evolved from the simplistic manually operated air-break switches used by Faraday and Henry into highly sophisticated vacuum contactors capable of handling enormous motor horsepower loads at voltages ranging up through 15 Kv.
Until the introduction of vacuum technology about 25 years ago, variations of air-break switches, breakers, magnetic contactors and combinations of these devices in numerous configurations were the only practical devices available to start and stop AC & DC rotating electrical machines. Vacuum technology offers an alternative to air-break devices by confining the arcing phenomenon to the interior of a sealed vacuum container. Vacuum technology offered so many inherent advantages over the old air-break method that it quickly became the technology of choice for controlling AC and DC rotating electrical machines of all sizes. Today, vacuum contactors are the predominant devices of choice in controlling the start-stop application of medium voltage motors (2.4 Kv-15 Kv) of all types.
About 40 years ago electrical researchers began experimenting with solid state motor starting techniques. This research explored various methodologies for starting AC and DC rotating electrical machines. A solid state motor starter uses electronic circuitry in place of the traditional contactors or switches to start and stop a motor. Dependable low voltage solid state starters for small motor loads became commercially available about 20 years ago and today are available for a wide range of motor loads and higher voltages. Inherent characteristics of the solid state starter are its low starting current requirements in bringing a large motor load up to full speed and its controlled soft stop capability when taken off line. This is especially advantageous in applications involving large medium voltage motors. These types of controllers are referred to commercially as a “solid state soft start”. High cost, sophisticated circuitry and technical limitations have traditionally restricted use of these devices to specialized applications. Solid state soft start controllers for large medium voltage motor applications are domestically manufactured by only a few specialty companies.
Within the solid state family of devices, controllers known as “variable speed drives” were an early product of solid state motor controller research. In addition to starting and stopping duty, these device also have speed control capability of the rotating electrical machine(s) during normal operation and start-stop cycles. These are hybrid devices using a sophisticated combination of solid state electronic magnetic or vacuum contactors and other components configured into a “control system”. They are available from numerous foreign and domestic manufacturers for a full range of low voltage through medium voltage motor applications. The high cost and the sophisticated technical nature of these systems restrict their use to highly specialized applications.
The present inventor is also the inventor of U.S. Pat. No. 6,208,111, issued on Mar. 27, 2001, for a motor starter arrangement with a soft start electronic control. This improved starter assembly is arranged, packaged and dimensioned so as to be substituted within an external profile of an existing motor starter. Each leg of the motor starter includes an electrical path which includes a main motor fuse and an in-line contactor and a soft start circuit. The soft start circuit includes a fuse, a voltage transformer, a current transformer and an electronic control system on a control card. The control card controls thyristors, along with a bypass contactor, in a conventional manner. The electronic control card controls the firing of the thyristors and the bypass contactor.
This patent utilized the commonly used thyristor as the solid state soft start controller. These electrically triggered thyristors are limited in reliability by the low di/dt and dv/dt and require the use of metal oxide varistors for protection against voltage surges, spikes or line faults. Metal oxide varistors pose a safety risk to personnel and equipment. These metal oxide varistors are explosive by nature and design. In addition, these electrically triggered varistors require localized firing boards in addition to fiberoptic firing interface boards. These electrically triggered varistors will have one line for the passage of voltage and current to the motor and another electrically actuated trigger line. In the wiring of the system, these lines will be placed in close proximity to each other. Surges from the main power line can create a “soft gating” of the thyristor based upon the noise in the main power line. As a result, it is somewhat difficult to electrically isolate the triggering electrical line from the main power line extending to the thyristor. The repeated soft gating of such thyristors can eventually damage the thyristors and will result in the replacement of such thyristors. In other circumstances, these electrically triggered thyristors would be destroyed upon receipt of power surges through either the triggering line or the main power line. For example, lightning strike in the vicinity of such lines could easily pass into the thyristor so as to cause a overload condition in the thyristor and eventually destruction of such electrically triggered thyristor.
The thyristor is a well known power semi-conductor switch that permits large currents to be switched at high voltages. The thyristors has four semi-conducting layers. Typically, the two outer layers are heavily doped extrinsic layers, while the inner two are lightly doped. Adjacent layers are oppositely doped from their neighbors, forming a number of semi-conductor junctions therebetween. The thyristor is turned on when carriers enter one of the inner layers. Typically, this is performed by injecting a small gate current pulse into one of the inner layers. Where the gate current is injected over only a portion of the inner layer, the current through the thyristor does not reach a maximum value until the entire layer is conducting. The time taken for the current to spread laterally to fill the layer is limited by the lateral carrier defusion velocity. The device only reaches full current capacity after the carriers have defused sufficiently to uniformly saturate the device.
Light-triggered thyristors have been developed to use optical triggering to actively switch on and/or off. Optical activation involves illuminating the semi-conductor device with light. The light is absorbed within the semi-conductor so as to produce electron-hole pairs at the site of absorption. Thus, optical activation permits the generation of carrier pairs within the device and does not require the injection of carriers. Hence, where the illuminating light pulse is short, optical activation can create carriers within the device considerably faster than injection, which is limited in speed by the carrier drift velocity. Optical activation can be used for switching semi-conductors on by a creating a population of carriers, for example, within a junction.
The judicious selection of the wavelength of the activating light results in the ability to control the absorption depth, and hence the volume of semi-conductor material activated by the optical pulse. This permits the fraction of wafer real estate devoted to activation to be reduced relative to that required for gate current activation, and hence the fraction of wafer real estate used for carrying the high power current is increased. The uniform illumination of the device give the possibility of combining fast turn-on times with large current carrying capacity.
One such optical-controlled thyristor is described in U.S. Pat. No. 6,218,682, issued on Apr. 17, 2001, to Zuker et al. This patent describes an optically controlled thyristor having a four layer thyristor structure with respective first, second, third and fourth layers. The first and third layers have a first doping type, and the second and fourth layers have a second doping type different from the first doping type. A first shorting structure, formed from a semi-conductor material of opposite doping from the first layer, is electrically coupled with the second layer by an electrically conducting, optically opaque layer. A first conductor layer connects between the first layer and the shorting structure and is adapted to transmit light into the first shorting structure. The first semi-conductor layer of an optically actuated thyristor has an aperture therethrough to permit light to enter the second layer from a first conductive layer side without proper gating within the first layer. The light source that is used for activating the silicon device is a Ndsup.3+ solid state laser. This laser has a laser line at a wave length of approximately 1.06 nanometers, which corresponds to an absorption depth in silicon of about 1 millimeter.
It is object of the present invention to provide an apparatus of method and method of soft starting a medium voltage polyphase motor with a solid state soft start controller utilizing laser diode firing, direct fiberoptic coupling isolation and light-triggered thyristors.
It is another object of the present invention to provide a method and apparatus for conversion of existing soft starters using electrically-triggered thyristors and metal oxide varistors into a solid state soft starter utilizing laser diode firing, direct fiberoptic coupling isolation, and light-triggered thyristors.
It is a further object of the present invention to provide greater thyristor reliablity.
It is a further object of the present invention to eliminate metal oxide varistors from the soft starters.
It is a further object of the present invention to provide greater electrical isolation from medium and high voltage sources.
It is a further object of the present invention to provide an apparatus which eliminates the phenomenon of “soft gating” which occurs in electrically-triggered thyristors.
It is a further object of the present invention to minimize the number of components used to trigger and protect the thyristors.
It is still a further object of the present invention to provide a smaller solid state soft starter than is currently available commercially.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.